1. Field of the Invention
This invention relates to an ignition monitoring circuit for an ignition system of an internal combustion engine of an automobile, etc.; more particularly, it relates to such an ignition monitoring circuit which outputs pulses of a fixed width when an ignition voltage is detected.
2. Description of the Related Art
Internal combustion engines operating on the Otto cycle, such as those used in automobiles, comprise an electrical ignition system. In the control of such internal combustion engines, the ignition voltages are detected by a ignition monitoring circuit (i.e. ignition signal detecting circuit), so that, for example, the supply of fuel may be stopped by the engine control unit when the ignition system is in failure. Such ignition monitoring circuits are generally required to output pulses of fixed width corresponding to the ignition voltages. In the case of widely-used conventional ignition monitoring circuits, however, there is a tendency that an erroneous pulse which does not correspond to any ignition voltage is outputted at the outset, when the key switch is made to connect the voltage source battery to the ignition system.
FIG. 1 is a circuit diagram showing the organization of such a conventional ignition monitoring circuit for an ignition system of an engine of an automobile.
The ignition system A, coupled to the battery 1 supplying the source voltage V.sub.1 through a key switch 2, comprises an ignition coil 3 and an ignition plug 4 coupled to the secondary winding of the ignition coil 3. The central processing unit (CPU), or more precisely a microcomputer, 5 outputs control signals to the power transistor driving circuit 6. The driving circuit 6 turns on and off the Darlington pair 7 consisting of a first and a second (i.e. power) transistor, 7a and 7b, to turn on and off the current through the primary winding of the ignition coil 3 coupled in series therewith, so that a high voltage may be induced across the gap in the ignition plug 4. The output of the driving circuit 6 is coupled to the base of the first transistor 7a of the Darlington pair to control the turning on and off thereof; the emitter of the first transistor 7a is coupled to the base of the power transistor 7b; and the collectors of the transistors 7a and 7b are coupled to a terminal of the primary winding of the ignition coil 3. Thus, the power transistor 7b is turned on and off in phase with the first transistor 7a.
The conventional ignition monitoring circuit comprises following portions: a pulse shaper circuit portion B coupled to the collector of the power transistor 7b, an RC circuit portion C of predetermined fixed rise time, a comparator circuit portion D, and an output circuit 20.
The pulse shaper circuit portion B, which detects the impulse voltages at the primary winding of the ignition coil 3 corresponding to the ignition voltages and shapes them into rectangular pulses of predetermined height, has the following organization. A voltage divider consisting of a serially connected resistors 8 and 9 is coupled across the point a at the collector of the power transistor 7b and the ground. A capacitor 11 and a Zener diode 12 in parallel circuit relationship are coupled across the junction between the resistors 8 and 9 and the ground through a rectifier diode 10 having the forward direction away from the junction between the resistors 8 and 9, wherein the positive electrode of the Zener diode 12 is directed toward the ground. Thus, the clamping Zener diode 12 limits the voltage at the junction between the resistors 8 and 9 under a predetermined level, i.e. the Zener voltage thereof. Further, a voltage divider consisting of serially connected resistors 13 and 14 is coupled across the negative electrode of the Zener diode 12 and the ground, whereby the junction between the resistors 13 and 14 constitutes the output point b of the circuit portion B. Thus, the pulse shaper circuit portion B shapes the impulses occurring at point a upon turning off the power transistor 7b into pulses of predetermined height.
The circuit portions C and D constitute together a constant timer circuit, i.e. a circuit for shaping output pulses of portion B into pulses of a fixed width. The circuit portion C comprises an RC circuit consisting of a serial connection of a resistor 16 and a capacitor 17 coupled across the battery 1 through the key switch 2. Further, a third transistor 15 having a base coupled to the output point b of the portion B is coupled across the capacitor 17 at the collector and the emitter thereof. The transistor 15 is turned on only when impulse voltages at the primary side of the ignition coil is detected, i.e. only when pulses are outputted from the pulse shaper circuit portion b at point b. The comparator circuit portion D, on the other hand, comprises a comparator circuit 18 having an inverting input coupled to the junction point c between the resistor 16 and capacitor 17 of the circuit portion C. The non-inverting input of the comparator circuit 18 is coupled to a constant voltage source V.sub.2 supplying a standard positive voltage thereto. The output of the comparator circuit 18, on the other hand, is coupled to the output circuit 20 through a resistor 19.
FIG. 2 shows the waveforms of the voltage Vi at point i supplying the battery voltage to the circuit portions A and C, and the waveforms of the voltages Va through Vd at points a through d, respectively, in the circuit portions A through D described above.
As shown at the top row (I) in FIG. 2, the voltage Vi at point i, coupled to the ignition coil 3 of the ignition system A and the resistor 16 of the portion C, rises abruptly from the ground to the battery voltage level V.sub.1 when the key switch 2 is made. After a length of time, when the driving circuit 6 starts to turn on and off the Darlington pair 7 at the commands of the microprocessor 5, high ignition voltages are induced in the secondary winding of the coil 3, so that impulse voltages are successively generated at the point a in the ignition system A, as shown by the waveform Va at second row (A). The clamping Zener diode 12, limiting the voltage at its negative electrode under the Zener voltage thereof, shapes the waveform Va at point a into a waveform Vb at point b consisting of rectangular pulses, with the help of resistors 8 and 9, rectifier diode 10, capacitor 11, and resistors 13 and 14.
On the other hand, the voltage Vc at the junction c between the resistor 16 and capacitor 17 rises from the ground to the battery voltage level V.sub.1 when the key switch 2 is made, as shown in solid line at the fourth row (C) in FIG. 2, since the transistor 15 is turned off at that time. Thereafter, each time a pulse of the waveform Vb is outputted from the circuit portion B, the transistor 15 is turned on to reduce the voltage Vc at junction point c to the ground level; the voltage Vc rises each time to the battery voltage level V.sub.1 with a fixed time constant determined by the resistance R of the resistor 16 and and capacitance C of the capacitor 17. On the other hand, a constant standard voltage shown by a dot and dash line V.sub.2 at the fourth row (C) in FIG. 2 is applied to the non-inverting input of the comparator circuit 18 when the key switch 2 is made, and the comparator 18 compares the waveform Vc with the standard voltage V.sub.2. Thus, the comparator circuit 18 outputs a pulse of a fixed width to the output circuit 20 through resistor 19 when the standard voltage V.sub.2 is greater than the voltage Vc (i.e. when the comparator circuit 18 is turned off). The voltage waveform Vd at point d, therefore, takes the form shown at the bottom row (D) in the figure.
Thus, the voltage waveform Vd consists of pulses of a fixed width each of which corresponds to an ignition voltage, except for the first pulse d1; the initial pulse d1, which is generated when the key switch 2 is made, does not correspond to any ignition voltage. Consequently, the conventional ignition monitoring circuit of FIG. 1 has the problem that an ignition signal is erroneously detected when the key switch is made to start the engine.